FIG. 1 shows a conventional Michelson interferometer. Light to be measured 11 is incident to a beam splitter 12 such as a semitransparent mirror, by which it is split into reflected light and transmitted light at a 1:1 power rate, and the reflected light is incident to a fixed reflector 13. The fixed reflector 13 is a reflector which reflects the incident light back to its incoming direction, such as a mirror or corner-cube prism. The transmitted light through the beam splitter 12 is incident to a movable reflector 14. The movable reflector 14 is also to reflect the incident light back to its incoming direction as is the case with the fixed reflector 13. The reflected light from the fixed reflector 13 and the reflected light from the movable reflector 14 return to the beam splitter 12, wherein they are combined and interfere with each other, and the resulting interference light is received by a photodetector and converted into an electric signal. The movable reflector 14 mounted on a linear motor 16 moves toward and away from the beam splitter 12. On the linear motor 16 there is mounted a linear scale 17 which extends in the direction of movement of the linear motor 16, and the movement of the linear scale 17 per unit length is detected by a linear scale detector 18. Its detected output is a pair of two-phase pulse signals D.sub.1 and D.sub.2 displaced 90.degree. apart in phase and the direction of movement of the linear scale 17 is indicated by which signal leads or lags in phase and one pulse is generated per unit distance. Such a combination of a linear scale and a detector is now commercially available. The two-phase output signal of the linear scale detector 18 is applied to a servo drive circuit 19. The servo drive circuit 19 has a microcomputer, which is supplied with a value of desired speed, desired distance and direction of movement of the linear scale 17 from a control circuit 21 and effects drive control of the linear motor 16 accordingly. Such a servo drive circuit is also commercially available. The linear motor 16 has a light blocking plate 22, and when the linear motor 16, i.e. the movable reflector 14 reaches a reference position, the light blocking plate 22 enters into a photo interruptor 23, which supplies the control circuit 21 with a signal indicating that the linear motor 16, that is, the movable reflector 14 is at the reference position. Setting the speed, distance and direction of movement of the linear motor 16 in the servo drive circuit 19 by the control circuit 21 in response to the signal from the photo interruptor 23, the servo drive circuit 19, to which the detected signal D.sub.1 and D.sub.2 from the linear scale detector 18 are fed back, servo-drives the linear motor 16 with a two-phase drive signal P so that the movable reflector 14 moves at a specified constant speed by a specified distance in a specified direction.
In the case of single-wavelength light, the intensity of interference light becomes maximum or minimum (zero) depending on whether the difference between the optical path from the beam splitter 12 to the fixed reflector 13 thence back to the beam splitter 12 and the optical path from the beam splitter 12 to the movable reflector 14 thence back to the beam splitter 12 is an even multiple (including 0) or odd multiple of the half wavelength of the light to be measured 11. Accordingly, when the movable reflector 14 is moved at a constant speed, the intensity of the interference light undergoes a sinusoidal change with a period corresponding to the wavelength. In the case of the light 11 containing a plurality of wavelength, a waveform containing frequency components corresponding to the wavelengths is detected by the photodetector 15 as the movable reflector 14 moves at a fixed speed.
In the conventional interferometer shown in FIG. 1, the distance and direction of movement of the movable reflector 14 are detected by the linear scale 17 and the movement of the linear motor 16 is controlled step by step or stepwise every quarter period of the detected two-phase signal; therefore, if the moving step of the movable reflector 14, that is, the length corresponding to the quarter period of the two-phase signal is large, the intensity of the interference light of the light being measured varies stepwise with the step-by-step feed of the movable reflector 14, resulting in undesirable modulation noise getting mixed into the signal which is detected by the photodetector 15.
To avoid such a problem, it is necessary to move the movable reflector 14 as smoothly as possible, i.e. with the shortest possible steps. This calls for a 1 .mu.m or less resolution of the linear scale 17 but such a high-resolution linear scale is appreciably expensive at present. Moreover, the use of such a high-resolution linear scale would require severe alignment in angle and position between it and the linear scale detector 18 and hence would involve troublesome adjustment therefor.